Arrowhead Subsidiary Successful in Small Animal Studies for Anti-Cancer Compound IT-101.

PASADENA, Calif. -- Arrowhead Research Corporation (NASDAQ:ARWR) (NASDAQ:ARWRW) announced today that its majority-owned subsidiary, Insert Therapeutics, has released data demonstrating effective anti-cancer results in animal studies of its lead product, IT-101, against various cancers, including pancreatic cancer, colon cancer, breast cancer, lung cancer, and Ewing's Sarcoma, a cancer primarily affecting children and young adults. In the case of two of the cancers, total tumor remission was achieved and the animals remained tumor free throughout the length of the 90-day study. Phase I/II human safety and efficacy trials are scheduled to begin at the City of Hope Medical Center upon clearance of regulatory requirements, expected later this year.

All animals with Ewing's Sarcoma and all animals with non small-cell lung cancer, which represents approximately 80% of lung cancers, experienced total tumor remission and remained tumor free throughout the duration of the 90-day study. IT-101 takes advantage of the "leaky" blood vessels of tumors to accumulate selectively in the tumors. Once inside the tumor cell, its slow release of the drug contributes to protracted anti-tumor activities that are substantially more effective at significantly lower doses.

"We are optimistic that this product will finally effectively treat some of the most difficult to treat cancers and provide a better and safer treatment for cancer in general," said R. Bruce Stewart, President of Arrowhead Research.

IT-101 is a conjugate of Insert's proprietary nano-engineered drug delivery vehicle, Cyclosert(TM) and the anti-cancer drug, camptothecin. Analogues of camptothecin, specifically irinotecan and topotecan, have nearly $1 billion in annual reported worldwide sales, despite the still serious side effects of these medications. In the study, anti-tumor activity in IT-101 treated animals was superior to irinotecan treated animals in all tumor models tested.

Dr. Thomas Schluep, Chief Scientific Officer at Insert Therapeutics presented the results of the animal studies at the American Association of Cancer Research Annual Meeting in Anaheim, Calif. on April 17, 2005.

About Arrowhead Research Corporation

Arrowhead Research is a development-stage nanotechnology company structured to bring together a diverse and innovative mix of technologies, rights to a broad suite of intellectual property, and some of the most respected minds in this dynamic field. There are three strategic components to Arrowhead's business model:

--The Commercialization Program: Arrowhead forms or acquires majority-owned subsidiary companies pursuing products based on nanotechnology in high-growth technology markets.

--The Research Program: Arrowhead funds nanoscience research at universities in exchange for exclusive rights to license the technology.

--The Patent Toolbox: Arrowhead acquires, licenses, and sublicenses intellectual property in nanotechnology. To date, Arrowhead has licensed

Arrowhead Research operates four majority-owned subsidiary companies:

--Aonex Technologies, Inc., developing and commercializing proprietary semiconductor nanomaterial technology.

--Insert Therapeutics, Inc., developing and commercializing a proprietary nanoscale drug delivery system.

--Nanotechnica, Inc., developing a variety of proprietary nanoscale devices and systems, including laboratory on a chip.

--Calando, Inc. is developing nanostructures for delivery and therapeutic use of RNA interference.

Arrowhead is funding three research and development efforts in nanotechnology at the California Institute of Technology.

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This news release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995. These statements are based upon our current expectations and speak only as of the date hereof. Our actual results may differ materially and adversely from those expressed in any forward-looking statements as a result of various factors and uncertainties, including the recent economic slowdown affecting technology companies, the future success of our scientific studies, our ability to successfully develop products, rapid technological change in our markets, changes in demand for our future products, legislative, regulatory and competitive developments and general economic conditions. Our Annual Report on Form 10-K and 10-K/A, recent and forthcoming Quarterly Reports on Form 10-Q and 10-Q/A, recent Current Reports on Forms 8-K and 8-K/A, our Registration Statement on Form S-3, and other SEC filings discuss some of the important risk factors that may affect our business, results of operations and financial condition. We undertake no obligation to revise or update publicly any forward-looking statements for any reason.